1. Field of the Invention
The present invention relates to non-aqueous electrolyte secondary batteries that use as a positive electrode active material a lithium-containing transition metal oxide containing nickel and manganese as transition metals.
2. Description of Related Art
In recent years, development of HEVs (Hybrid Electric Vehicles), which use electric motors in conjunction with automobile gasoline engines, has been in progress worldwide in order to resolve the environmental issues caused by vehicle emissions. Nickel-hydrogen secondary batteries have conventionally been used as power sources for the HEVs, but lithium-ion secondary batteries have been expected to be applied as HEV power sources because of their higher voltage and capacity.
One of the important issues relating to a lithium-ion secondary battery for HEV applications is to reduce the costs. Lithium-ion secondary batteries that have already been commercially available for power supply applications of portable electronic devices such as mobile telephones, camcorders, and notebook computers generally use a composite oxide containing Co as the positive electrode active material. However, because of cost considerations, positive electrode materials that do not contain costly metal elements such as Co are desirable for the large-sized lithium-ion secondary batteries for HEVs. For HEV applications, higher input power is preferable from the viewpoint of system design particularly for the purpose of efficient battery regeneration. Accordingly, a battery with low charge-discharge voltage is needed, and in addition, a battery that achieves a good balance between input power and output power is desirable. In particular, in HEV applications, not all the capacity range of the battery is evenly used but the charge range in the vicinity of 50% SOC is mainly used. Therefore, the design requirements are that the battery should have low charge-discharge voltages in that range and exhibits a good balance between input power and output power. A problem with conventionally used active materials, such as lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium-manganese oxide (LiMn2O4), and Li—Ni—Co—Mn composite oxide, is, however, that these materials cause the positive electrode potential to be high, and thus lead to high battery voltage, resulting in low input power. In view of such circumstances, a low-cost, low-voltage lithium-ion secondary battery designed to exhibit excellent power characteristics is sought after for HEV applications.
In recent years, active materials that are made of only elements that can be obtained at relatively low cost, such as lithium-containing olivine-type phosphate and Ni—Mn-based composite oxide, have been investigated widely as positive electrode materials for lithium-ion secondary batteries for HEV applications that can meet the above-mentioned requirements. Among them, a Li(Li—Ni—Mn) composite oxide having a crystal structure that belongs to the space group R-3m and containing lithium at the transition metal site enables the charge-discharge potential at 50% SOC to be about 100 mV to 200 mV lower than those of the above-described positive electrode materials that have already been in commercial use, because lithium extraction from the 3b sites occurs during an initial charge at 4.45 V (vs. Li/Li+) or higher and, after this reaction, the capacity originating from the oxidation-reduction reaction of Mn3+/Mn4+ is obtained at 3.5 V (vs. Li/Li+) or lower. Because of these properties, the Li(Li—Ni—Mn) composite oxide has drawn attention as a promising positive electrode material that achieves high capacity and higher input power at low cost. (See, for example, U.S. Patent Application Publication No. 2003/0108793A1).
Nevertheless, the Li(Li—Ni—Mn) composite oxide disclosed in the just-mentioned publication shows a high irreversible capacity during the initial charge, so the initial charge-discharge efficiency of the positive electrode active material is poor. This necessitates excessive use of the negative electrode active material in the battery design, which leads to problems such as a low battery capacity and poor load characteristics.